There are many applications in which it is necessary to be able to assure continuous flow of fluid from a container under varying conditions of acceleration, ranging from zero gravity space conditions to the high acceleration experienced in high performance aircraft and missile systems. In view of the corrosive nature of some of the materials which are normally used as fuels or oxidizers such as for example, nitrogen oxides or hudrazine, the state of the art devices generally consist of a pressure vessel containing a collapsible metallic bladder which, in turn, contains the fluid to be expelled. The bladder is caused to collapse by the application of a high-pressure gas between the inside of the pressure vessel and the outside of the bladder. In order to get maximum expulsion efficiency from such devices, it is necessary to control the collapse of the bladder in a predetermined manner to avoid, as much as possible, the development of unpredictable folds, wrinkles, or other random collapse patterns which could result in the uneven and incomplete collapse of the metallic bladder. While various approaches have been taken to this problem, one of the most promising approaches and the one to which this invention most closely relates, is the so-called three-lobed positive expulsion device, such as for example is illustrated by U.S. Pat. No. 3,404,813, Waxman, Tank With Metallic Bladder, dated Oct. 8, 1968, U.S. Pat. No. 3,720,389, Ferris, for Expulsion Bladder, dated Mar. 13, 1973, or described in the Technical Report for the Air Force Rocket Propulsion Laboratory under Contract F04611-76-C-00161, dated March, 1977, titled "Technical Report for July 19, 1976-March 19, 1976, Advanced Post-Boost Vehicle Propulsion Fuel System, AFRPLTR-77-36," particularly pages 12-39, 107-115, 124, 127-132. As can be seen from these documents, these devices consist of a pressure vessel and bladder as described above. Inside the bladder is mounted a three-lobed support structure around which the metallic bladder collapses upon the application of the pressurizing medium. The internal framework substantially increases the degree of predictability of the collapse pattern of the bladder, but even with this support structure it has been found necessary to vary the thickness of the bladder by chem milling, or otherwise as is suggested in U.S. Pat. No. 3,722,751, Bisciglia, for Control Fold Liquid Expulsion Bladder, dated Mar. 27, 1973, or shown in the AFRPL report, so that the collapse of the bladder can be initiated in predetermined areas and will propagate in a more predetermined manner. Accordingly, the collapsing bladder positive expulsion device prior art of which the inventor is aware can be fairly represented as utilizing a combination of separate internal support frameworks and a variable thickness bladders to produce positive expulsion devices having high degrees of expulsion efficiency. While such devices work reasonably well, there are certain problems associated with the use of the internal support structures. First of all, these support structures have some weight and occupy some volume and therefore decrease both the mass loading and the amount of fluid that can be carried. Further, additional parts, attach structures and weldments are necessary, all of which must be inspected and are subject to failure under vibration and acceleration loads which decrease the reliability of the system. Obviously, it would be desirable to eliminate the separate support structure, but it has heretofore not been found possible to do so and at the same time, retain the degree of control of the collapse of the three-lobed bladder that is necessary. In fact, in all such prior systems of which the inventor is aware, it is the support structure itself which defines the pattern of collapse of the bladder. According to this invention, however, I have devised a multi-lobed positive expulsion device which collapses in a predetermined pattern without any separate support structure, thereby avoiding all the inherent disadvantages of the support structures which are set forth above.
Accordingly, it is an object of this invention to provide a multi-lobed positive expulsion device which does not employ a separate support structure. It is a feature of this invention to form, integrally in the bladder portions having higher stiffness and resistance to collapse whereby the bladder itself performs the supporting functions otherwise performed by the separate internal support structure.